Not being an engineer myself, though finding this a very interesting subject, I had a chat with a mate of mine the other night over a bottle of wine or three. This guy is responsible for the final approval of the turbine disks in lots of the planes you guys fly, so knows his metallurgy, he doesn't claim to be an expert on composites, but this is what he emailed me today.... What do you say to this Belgique?
"I agree structural composites are a bad idea because repairs are difficult and the lack of toughness of composite materials. I have never seen a repair scheme for a structural composite, because it is not wise and there are few structural composite parts out there. Typically, to manufacture a composite there is the fabric, honeycomb, resin and tooling. The process requires vacuum bagging and a moderate temperature cure cycle with ventilation and plumbing to maintain the vacuum as the bagged composite assembly outgasses - not possible for an in-situ repair. Beyond that composites lack toughness. They have strength to weight ratios greater then metal, but they do not yield. Composites support a load, with little elongation, until failure, which is usually catastrophic - I am not sure what their response to fatigue loading is or their crack propagation characteristics. Because metals are loaded to a fraction of their 0.2% yield strength, they are somewhat easier to design with.
I suspect the tail was composite as a last resort to save weight. The inspection practice for the tail, probably ultrasonic, was insufficient to pickup material degradation. Ultrasonic inspection will pickup voids, but if you have delamination without a void, the sound will pass through the assembly without an echo, and the part will pass the inspection. Other inspection techniques, eddy current or fluorescent penetrant inspection, also require a void of some sort, near surface crack tip or inclusion, or porosity open to the surface.
I believe Airbus have incorporated many technologies to save weight and cost including structural composites and structural casting. But for this there is some risk. Most likely Airbus will have to reevaluate the risk associated with its business model as a result of the accident."